* class DataLength(dfb.DataFilter) cf. CountBytes
* Use tank_queue for calculating vertical height change rate
* Make ">>>" redundant, since it adds nothing to the logic of
and just complicates things because of when it is required.

See the **TO-DO** markers in filterpype/*.py for a complete list.

Credits
=======

FilterPype is written and maintained by Flight Data Services, Ltd
<developers@flightdataservices.com>.

Substantial parts were written by Rob Collins
<rob.collins@pythonpro.co.uk>